Cobalt-iron-tantalum high-temperature-strength alloy

ABSTRACT

This invention covers a cobalt-base alloy consisting essentially of, in atom percent, 8-12 percent iron, 0.4 to 1.7 percent tantalum, and the remainder cobalt.

United States Patent Inventors Fred C. ltohertlhaw Clnclnnati;

Jon L. Bartos, Loveland, both of Ohio 4,375

Appl. No.

Filed Jan. 20, 1970 Patented Sept. 21, 1971 Assignee I H The United States of America as represented by the United States Atomic Energy Commission COBALT-IRON-TANTALUM HIGH- TEMPERATURE-STRENGTH ALLOY 1 Claim, 4 Drawing Figs.

us. (:1 7 5/170, gags, 148/158 Primary Examiner-Richard 0. Dean Attorney--Roland A. Anderson ABSTRACT: This invention covers a cobalt-base alloy consisting essentially of, in atom percent, 8-12 percent iron, 0.4 to 1.7 percent tantalum, and the remainder cobalt.

CREE? OF 3.2 Kg. VIM C BASE ALLOYS AND TYPE 316 STN. STL. AT 750C AND i050 Kg/Cmz (1500psi) IN ARGON E H RUPTURE x(( x CURVE couwosmou TIME HRS. ELON6.%

J A Fe'iBCi'MNi'lSMo 87.5 39.5 44 a Co-9Fe-0.4To 71.9 20.?

l e c Co9Fe-i.7To 574.0 194 D Co-9Fe 0.7 25.2 3 i E Co-9Fe-2Mo 3.2 74.3 F Co-9Fe-1Ti -45 53.7 0 Co-9Fe-4Ti 314 01.5 Lu 7 a 11 Co-9Fe0.5Nb 21.5 77.3 x I Co-9Fe-2Nb 15.5 64.4 7 .1 Co-9Fe-1T11 104.4 459 g j B K Co9Fe-O.8T11 573.0 1.0 g l 1. Co-SFe-QSTo 4oa3 1.8T u a T=TEST TERMlNATED-NO RUPTURE x= RUPTURE T 4 a iii TIME HOURS COBALT-IRON-TANTALUM HIGH-TEMPERATURE- RE GTE LL Y BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the U8. Atomic Energy Commission.

The present invention relates to high-temperature-strength cobalt-base alloys having a substantially all-faccvcentered cubic (alpha) microstructure which is stable from room temperature up to the melting point.

In designing a cobalt-base alloy for use at high temperatures it is desirable to maintain the face-centered cubic structure throughout the temperature range from room temperature to in excess of the intended service temperature in order to eliminate the compromising alpha-to-epsilon phase change characteristic of elemental cobalt.

It is known that certain substitutional alloying additions to cobalt can promote or stabilize the face-centered cubic (a) phase. However, for maximum benefits, the alloy must not only exhibit phase stability, but should have a satisfactory combination of short and long term (creep) strength as well as ductility at room temperature as well as at high temperatures; specifically, a temperature in the range 550750 C.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a cobalt-base alloy system which not only does not undergo phase transformation over a wide range of temperature, but which has a satisfactorily high combination of short term and long term strength to make the alloys particularly useful for service conditions at a temperature in the range 550-750 C. This twofold object is met by providing a cobalt-base alloy containing sufficient amounts of iron to ensure the required alpha phase stabilization and sufficient amounts of tantalum to provide a practical combination of improved strength and ductility, rendering the alloy useful at temperatures in excess of 500 C. and as high as 750 C.

An examination of the cobaltiron equilibrium phase dia-. gram found in the literature will reveal that small amounts of iron will form a binary alloy stable in the alpha phase. In the} present invention we find that from 8 to 12 percent iron will guarantee the existence of austenitic phase throughout the; range of temperature from room temperature up to the meltlng point.

The present invention is predicated on the discovery that tantalum added to an alpha-stabilized cobalt-iron reference alloy combines to form a ternary-base system with a satisfactory combination of beneficial high-temperature properties. Ad-

dition of tantalum in stipulated amounts will provide a ternary alloy which is easily fabricable and as stable as the reference cobalt-iron binary system, but which has significantly increased short term and long term tensile properties at temperatures in the range 550-75 The fllgy system possessing this unique combination of properties is one consisting essentially of, in atom percent, 8 to 12 percent iron, from 0.3 to 2 percent tantalum, and the remainder cobalt. By the term consisting essentially of" we mean to include alloying elements positively recited which critically determine the physical and mechanical properties of the alloy as well as residual amounts of other elements which are not deliberately added for any specific alloying purpose but which, nevertheless, accompany the alloy heat during its formation and the wrought alloy during its fabrication into a desired part or shape. In

general terms, such residual amounts of unstated impurities do not exist at more than the parts-per-million range with an approximate upper limit of no more than about 50 to parts per million for each residual element.

The addition of tantalum to the reference cobalt-iron alloy to produce the aforementioned combination of satisfactory qualities is surprising from a number of standpoints. While recent work has indicated that tantalum raises the transformation temperature of cobalt, we find that it does not reverse the stabilizing effect of iron. Moreover, the addition of tantalum within the prescribed limits produces pronounced enhancement both in the short term and long term, i.e., cr p, Strength relative to the reference cobalt-( 8-12) iron alloy. This effect is unique when it is considered that, while other ternary additives may be used to increase the short term strength of the binary alloy such as titanium, niobium, or molybdenum, they do not enhance creep strength to any satisfactory degree. Finally, the addition of tantalum within the prescribed limits provides an alloy which is easily prepared either by powder metallurgy or casting techniques. We prefer to form the alloy by vacuum casting in alumina crucibles under a vacuum of less than 1 micron because it leads to closer control of and reduces the amount of residual impurities. After the cobalt and iron in the crucible become molten, a partial pressure of argon is introduced, tantalum is added, and after a suitable molding period the resultant heat is poured into a copper mold. In our testing program all heats were extruded to 1.9 centimeters in diameter and converted to 0.076-centimeter thick sheet at rolling temperatures from 760 to about l,200 C. Heating for forging, extrusion, and/or rolling operations were conducted in argon atmosphere. Tensile specimens were then prepared for testing. The crystal structures of all the cobalt-base alloys which were processed to form the tensile specimens were determined by X-ray diffraction techniques to be face-centered cubic.

The following description provides a summary of specific physical test data which characterize the novel Co-Fe-Ta system previously described in general terms.

MATERIAL EVALUATION Tensile Properties Tensile testing was made on 0.076-cm. thick sheet specimens of a number of cobalt-iron-tantalum alloys in the as-rolled-and-aged (750 C.20 hr.) condition. The results are indicated in the table below.

TABLE Tensile properties a of 3.2 KgVlM Co-base alloys Test Yield strength b Tensile strength emp.,-= Elong., Alloy beat No. Composition, a/o C. K s.i. KgJcm. K s.i. KgJcm. percent M433 00-9Fe d RT 18.7 1, s10 70. 7 4, 950 s4. 1 550 8. 9 620 43. 8 3, 060 44. 1 650 8. 6 600 24. 9 1, 740 36. 0 750 7. 8 650 17. 4 1, 220 24. 5

M-406 C09Fe.0.4Ta RT 34. 1 2, 390 71. 2 4, 980 i 68. 7 550 20. 0 400 40. 3 2, 821) 25. 5 650 25. 3 1, 770 35. 3 2, 470 16. 7 750 23. 8 1, 670 28. 2 1, 970 34. 2

M-446 Co9Fe0.6Ta RT 55. 0 a, s50 88. 2 e, '-44. a 550 44. 2 a, 100 as. 0 a, 920 is. 1 650 42. 7 2, 990 51. 0 3, 570 20. 6 750 38. 0 2, 660 41. 6 2, 920 19. 9

M-445 Co9Fe-0.8Ta RT 55. 7 3, 900 90.4 6, 320 48. 9 550 41. 5 2, 900 58. 3 4, 080 16. 6 650 47. 4 3, 340 56. 6 3, 960 17. 9 750 42. 8 3, 000 47. 8 3, 340 17. 1

TABLE Continued Test Yield strength Tensile strength t Alloy heat No. Composition. alo C. K s.i. Kg./cm.'-' K s.i. Kg./cm.-' percent M423 C-9Fe-1Ta RT 90. 2 0, 310 121. 7 8, 520 28. 2 550 67. 0 4, 690 78. 4 6, 480 11. 0 650 64. 6 4, 530 70. 8 4, 950 1b. 0 750 53. 0 3, 710 58. 2 4, 070 20. 0

M409 Co-9Fe--l.7Ta RT 91. 7 6, 420 128. 3 9, 22. 2 650 76. 3 5, 340 91. 6 6, 410 7. 2 660 71. 5 5, 000 82. 5 5, 780 6. 6 750 65. 2 4, 560 71. 7 5, 020 5. 4

Type 316 SS- RT 35. 7 2, 500 83. 2 5, 830 72. 5 550 16. 0 1, 120 64. 0 4, 470 42. 5 650 13. 8 970 47. 9 3, 350 47. 1 750 13. 0 910 34. 0 2, 380 52. 2

- Specimens were 0.076 cm. thick with a 0.63cm.-wide reduced section and a 2.54pm. gage length. The major axis of each specimen parallels the rolling direction. (except where noted).

All specimens were annealed at 750 C. for hours in argon 11 Yield strength value is based on deflectometer measurement of total load train elongation and is approximate.

6 All elevated-temperature testing conducted in argon. d RT=room temperature.

Results show that all the ternary additions of tantalum are particularly effective in increasing the short term tensile strength of the reference Co-9Fe binary alloy. While the increase in strength effected by increasing the tantalum content is accompanied by a reduction in ductility, it is clear that a favorable balance of strength and ductility can be achieved within the range of 0.4 to 1.7 atom percent tantalum. It should be noted here that ternary additions of such metals as Nb, Mo, and Ti will also increase short term tensile strength of the resultant ternary alloys as compared to a reference Co-9F e alloy, but, as will be seen, do not improve creep properties nearly as well as a tantalum addition.

Creep Rupture Properties The vacuum-induction-melted cobalt-base alloys previously referred to were creep tested in the as-rolled-and-aged (750 C.20 hrs.) condition at 650 C. with a load of 1960 kgJcm. and at 750 C. with a load of 1,050 kg./cm. in argon atmosphere. The results are illustrated in FIGS. 1 and 2, which reveal that the cobalt-iron-tantalum alloys are far superior in creep to those alloys which have niobium, titanium, and

molybdenum as the ternary additives. For purposes of comparison, the creep curve for a 316 stainless steel (curve A) is also included since this alloy can be used at temperatures in the range 550 to 750 C. It is clear that the cobalt-iron-tantalum alloys are superior to 316 stainless steels at both temperatures.

Age-Hardening Effect During the course of processing and testing of the cobaltiron-tantalum system of the present invention we have discovered that additions of tantalum impart hardness upon suitable heat treatment involving a preliminary solid solution treatment, quenching, and final aging at a lower temperature. The results show that heat treatment at a temperature in the range 1,200 to 1,210 C. for 1 hour will dissolve virtually all of the secondary phase materials. Sheet specimens were then aged at temperatures in the range 550 to 750 C. for 1 hour and it was found that maximum hardness was achievable at an aging temperature of650" C. for a period of about 50 hours. Hardness values then remain fairly constant. The change in hardness resulting from a typical solutioning and heat treatment at 650 C. for 50 hours is shown in the curves of FIG. 3. As with tensile strength it is seen that there is a direct relationship between the increase in hardness and the tantalum content of each alloy. I

The effect of age-hardening in accordance with the solid solutioning-aging two-step operation results in the formation of a secondary phase which has been identified by metallographic and electron microscopy studies as forming a fine precipitate randomly distributed with thin grains and agglomerated in the grain boundaries. This secondary phase has been identified as cobalt-tantalum particles, yco Ta, a predominant portion of which are in the range of about to 200 angstroms in diameter. it is this secondary phase which is believed to impart additional strength and hardness to the ternary cobalt-iron-tantalum system as compared to the reference binary cobalt-iron alloy.

in summary, we have shown that the addition of tantalum to a reference cobalt-iron ternary alloy imparts a unique combination of strength-inducing qualities to the new ternary cobalt-iron-tantalum system. The new alloy is characterized in having a stable, face-centered cubic, crystallographic phase throughout the range of temperature from room temperature to melting, avoiding any difficulties involved in phase transformation. In addition, the alloy shows a favorable combination of short term and high-temperature tensile strengths combined with excellent creep resistance as well as imparting an increased measure of hardness to the reference cobalt-iron alloy. These improved physical qualities are apparently due to the formation of a stable cobalt-tantalum secondary phase uniformly distributed within a gobalt-iron matrix The improvement obtained in this new cobalt-iron-tantalum system has already been demonstrated in the foregoing presentation of short term tensile, creep, and hardness data. They are somewhat more graphically illustrated in the curve of FIG. 4, which is a Larson-Miller parametric representation of the rupture life of several cobalt-iron-tantalum alloys compared with a 316 stainless steel, an alloy which can be used at temperatures in the range 550 to 750 C. The curves very clearly show that the Co-9Fe-Ta alloys containing 0.4 to 1.7 atomic percent tantalum are more creep resistant than 316 stainless steel for high-stress, that is, from 25 to 50 k.s.i., hightemperature (550 to 700 C.) applications. Lower stresses up to 25 k.s.i. at higher temperature, 700 to 800 C., applications require cobalt-iron-tantalum alloys with tantalum contents of 1 percent or more in order to exceed the rupture life properties of 316 stainless steel.

All concentrations of the novel alloy disclosed in this specification are in terms of atom percent.

We claim:

1. A cobalt-base alloy consisting essentially of 8-12 percent iron, 0.4 to 1.7 percent tantalum, and the remainder cobalt. 

